Refurbishing the Wavetek SAM for 88–108 MHz

The Wavetek SAM is a cable-TV test instrument introduced in 1980. The design evolved through many iterations and early models are cheap. This writeup covers the original SAM I and the SAM II.

SAM means signal analysis meter. The device is a calibrated, battery operated receiver that tunes 4 to 300 MHz. (One option increases coverage to 450 MHz; another adds the 470–890 MHz UHF-TV band.) Its primary purpose was to measure the amplitude of cable-TV signals. It has a wideband RF attenuator with 10-dB steps and a precision level meter calibrated in 1-dB steps over a 20-dB range. The minimum measurable signal level is −40 dBmV and the maximum is +60 dBmV (10 µV to 1 V). A small speaker and volume control help identify signals. The SAM I has analog tuning in multiple bands with a calibrated circular dial. In addition to signal level, the unit can measure residual hum (unmodulated test signal required), S/N (a two-step narrowband measurement calibrated for 4-MHz NTSC video bandwidth), and AC or DC voltage on a 100-V scale. A 150-MHz oscillator provides a +20 dBmV calibration signal with a specified accuracy of ±0.25 dB.

The most interesting secondary function is the spectrum analyzer, where the SAM generates horizontal and vertical signals for an external oscilloscope. The display is logarithmic with adjustable frequency span and sweep rate, or with manual scanning. It's fascinating to hook up a scope and watch the FM spectrum change in a complex way as you rotate an antenna. The span can be narrowed to a single signal or widened to cover an immense amount of spectrum. For someone without one, the spectrum analyzer may be the unit's most attractive feature.

The spectrum analyzer is an option, but most units seem to include it. Those without it use hole fillers as shown above.

I got the SAM primarily for aiming fixed rooftop antennas. It can measure signal levels even from low-gain antennas. I soon discovered that the SAM also was useful for aiming rotary antennas. It has much better resolution than an LED signal-strength indicator and won't saturate on strong signals as a tuner's analog meter will. It permits very accurate antenna aiming. I was surprised how poorly an LED indicator performed when aiming a small Yagi. It was easy to pick up a dB or two using the SAM.

With the SAM you can aim antennas, accurately compare antenna gains, precisely measure antenna patterns, check the axial ratio of a circularly polarized array, measure feedline, balun, or power splitter losses, and determine the gain of antenna or distribution amplifiers. You can use broadcast signals for any of these measurements. With a signal generator and power splitter, you can measure antenna or tuner return loss and then calculate mismatch loss. Resolution to a couple tenths of a dB is easy. Absolute accuracy is rated as ±0.5 dB at 70° F, or ±1 dB from 0°–120° F when calibrated at the operating temperature.

The hand-drawn, low-frequency calibration correction curve inside the top cover is a nice touch. Another is the way the unit turns itself off. Inside the lid is a storage compartment that opens with a spring-loaded knob. When you close the lid, the knob pushes the front-panel power switch to the off position.

To open the SAM for battery replacement, modification, or alignment, remove the three screws on the bottom.

Detection

The IF strip is linear with AGC applied to the first stage. Both FM and AM detectors drive the audio output. AM detection enhances multipath distortion, which can be useful when aiming an antenna. AM lets you hear NTSC video buzz and ATSC noiselike modulation, and it lets you copy signals in the 108–137 MHz aircraft band. The portable unit is handy for locating power-line noise since AM detection makes it quite audible. With a short wire antenna I was able to hear WWV at 5, 10, and 15 MHz as well as many shortwave broadcast stations. I was surprised that the wide IF didn't jumble several signals together, but the loudest stations came through clearly. The dial is calibrated down to 4 MHz but actually tunes to DC. I could receive many stations in the 540–1700 kHz AM broadcast band at reduced signal strength.

Selectivity

The IF uses LC networks and a single 10.7-MHz ceramic filter. The stock filter is a 280-kHz Murata. While selectivity was adequate for strong signals, I found it difficult to measure weak signals on adjacent FM channels. I unsoldered the filter, installed a socket, tried several substitute filters, and finally settled on a 110-kHz Murata. I compensated for 2 dB of additional loss by readjusting the IF-gain pot.

This shows a ceramic filter in its socket. The IF-gain pot is at the lower right. The slide switch breaks the IF AGC loop for testing, substituting a fixed AGC voltage determined by the pot above it. I have yet to find a need for this feature.

This is the spectrum analyzer display of the FM broadcast band using the original 280-kHz filter.

This is the response using the 110-kHz filter.

An asymmetrical passband shape may be due to the ceramic filter, or the filter may be on the response slope of the other IF tuned circuits, which are resonated with factory-selected capacitors. Once you've socketed the filter and aligned the unit as described below, it's easy to check. Run the spectrum analyzer on the calibrator signal, narrow the frequency span with DISP (dispersion) until the filter shape is apparent, and then substitute a .01-µF ceramic capacitor for the filter.

Fine Tuning

In the FM band the FINE TUNE control originally spanned 8 MHz. This wasn't much help in tuning FM signals. Adding 100kΩ resistors to the end terminals of the 25kΩ pot reduced the fine-tuning range to about 900 kHz.

Battery

The battery pack consists of 12 sub-C NiCd cells of unmarked capacity. The charging current is about 170 mA for a partially charged battery, dropping to about 155 mA at full charge. The circuitry draws a nearly constant 160 mA; the light adds 20 mA. The manual specifies an 8-hour operating time, which suggests an effective battery capacity of about 1300 mAH. Charging the battery with the unit on results in a net inflow or outflow of a few milliamperes, depending on battery charge.

When the NiCds died I installed 2100-mAH NiMH AA cells to provide longer operating time and lower weight. Three four-cell AA holders just fit inside the plastic sub-C enclosure, making conversion easy.

With higher-capacity batteries it would be tempting to increase the charging current to reduce recharge time. However, the small line transformer in the charging circuit already gets slightly too hot to touch. The transformer feeds a bridge rectifier and 1000-µF capacitor. The 20 V of DC developed charges the battery through a #1891 bulb paralleled with a panel lamp to provide constant current. Circuit simulation suggests that lowering the peak winding current by disconnecting the capacitor, adding a resistor across the bulbs, and charging with pulsating DC averaging up to 250 mA will not increase transformer temperature.

The battery recharge point on the meter originally corresponded to 12.9 V (1.075 V/cell). I found that the regulated voltage (11.93 V in my unit) did not drop until the input voltage had fallen to 12.1 V. To make this the indicated recharge point, I installed a 360kΩ resistor across the 24.9kΩ 1% meter resistor. 1.0 V/cell is the typical cutoff voltage recommended by NiMH battery manufacturers so this worked out nicely. In another SAM the original recharge point was 12.7 V and dropout occurred at 12.3 V. 680kΩ was required across the meter resistor, which in this unit was a series combination of 5% resistors.

If the cell capacities differ much, one or more cells may completely discharge and become reverse-biased before the battery voltage reaches the recharge point. This can shorten cell life. If you see the meter suddenly drop when checking battery voltage, often a few seconds after applying power, it's time to recharge regardless of what the meter says.

Alignment

The SAM has dozens of adjustments. I experimented with all except those in the front-end module.

The rectangular metal can with colorful slugs is the FM discriminator.

The red dots locate adjustments on the underside of the board. I'll identify these left to right as upper group U1–U2, middle group M1–M6, and lower group L1–L6. Here's what they do:

There are two additional adjustments not visible in the photo. The slug within the calibrator module sets its frequency, while the trimpot beside it sets its output level.

It's best to align the unit at the temperature at which it will be used. Turn it on and wait until the tuned frequency stabilizes. Then adjust things in the following order.

Center CALIBRATION SET and FINE TUNE. Set FUNCTION to SLM (signal level mode), GAIN to CAL, and turn AFC on. Set ATTENUATOR so that +15 and +25 show in the meter windows. Connect a signal generator at about −30 dBm to RF IN and tune the main dial to peak the signal. Then simultaneously adjust L6 and FINE TUNE for maximum meter reading. Next, simultaneously adjust the blue discriminator slug and FINE TUNE for maximum. Finally, adjust the pink discriminator slug for maximum.

Switch AFC off and adjust U2 to repeak the signal.

Turn AFC back on and peak the signal with FINE TUNE. While switching the signal generator step attenuator over three 10-dB steps, adjust M5 and L1 for best accuracy over the 20-dB meter range. L2 and L3 also affect accuracy by changing the IF amplifier operating point. If necessary, adjust them as well.

Turn GAIN to +10dB and adjust M4 for best accuracy over the 20-dB meter range. The two gain settings interact somewhat. Readjust M4, M5, L1, L2, and L3 for best accuracy at both settings. With the meter needle aligned at the leftmost and rightmost scale marks for signal levels 20 dB apart, I was able to get the midscale calibration within 0.2 dB for both gain settings.

Switch to spectrum analyzer mode and apply a signal with AFC off. Adjust M2 and M3 for best vertical linearity by switching the attenuator over a wide range. I was able to linearize the upper 40 dB or so, with a little expansion in the next 20 dB.

L2 may cause the trace to saturate in spectrum analyzer mode. Either readjust L2 until the trace unsaturates and then redo the signal level mode alignment, or change the value of the factory-selected resistor in series with M3. M3 should be able to override any L2 setting, but usually I find that it can't.

Connect a frequency counter to CALIBRATION OUT. Adjust the slug within the calibrator module for 150 MHz. Then connect a 75Ω RF voltmeter or power meter and adjust the trimpot next to the module for +20 dBmV (−28.8 dBm). Be sure to account for the loss of the measurement cable.

Jumper CALIBRATION OUT to RF IN with a cable no longer than a few inches. Switch to signal level mode, tune to 150 MHz, set the attenuator so that +15 and +25 show in the meter windows, set GAIN to CAL and AFC on, and peak the meter with FINE TUNE. Then adjust CALIBRATION SET so that the meter is at midscale. If the control has insufficient range, adjust L3 until it does and then redo the signal level mode alignment without changing L3.

In signal level mode the video signal (detected AM) appears on VERT. Apply a 100%-modulated, 1-kHz, AM signal and adjust M6 so that the waveform does not clip high or low.

The vertical board provides trimpots to calibrate the dial. The six pots on the left align the low end of each band. The pots follow the order of the rotary bandswitch. The six pots on the right, somewhat deeper, align the upper ends. Center FINE TUNE and turn AFC off. Pick frequencies near the lower and upper band limits and adjust the pots using an accurate signal generator. The two adjustments interact somewhat. Check the calibration near the center of each band before moving on to the next. Dial accuracy is specified as ±1 MHz.

With the UHF option, bands A-E cover 470–890 MHz when the switch is at UHF. The vertical board contains ten additional trimpots, recessed and in the same order as the VHF pots. The little red squares around the dial edge mark UHF video-carrier frequencies, while the red numbers at the bottom identify the UHF channel range for each band. What a neat way to pack so much information onto a single small dial. I recalibrated so that the red squares mark channel centers. This seems best for ATSC signals.

SAM II

The SAM II is similiar to the SAM I except for the LCD frequency readout and TV-channel keypad. My unit had a maximum frequency of 300 MHz. A single tuning knob covers the entire frequency range in ten turns; there is no fine tuning control. Frequencies are displayed to 0.1 MHz when tuning manually and 1 MHz when using the keypad. There is no way to enter an arbitrary
frequency with the keypad. Both the meter and LCD have lamps.

My SAM II had a retrofitted 12-V, 2300-mAH, sealed lead-acid battery. The charging circuit was unmodified. The battery recharge point on the meter corresponded to 12.2 V. Right at this point the regulated voltage (11.98 V in this unit) began to drop. Panasonic literature indicates that a 12-V sealed lead-acid battery has 20 to 50 percent of its capacity left at 12.2 V at room temperature. Therefore the effective capacity of the 2300-mAH battery in my SAM II was somewhere between 1200 and 1800 mAH.

The vertical board in the SAM II is the frequency synthesizer/counter. It uses common CMOS ICs, all socketed. Two trimpots labeled G and O are coarse and fine VCO adjustments. When misadjusted the PLL may not lock on every keypad channel. About an inch to the left of G is a wire loop. Attach a scope probe and adjust G and O for a stable trace when punching up all channels.

With the keypad I brought up a local NTSC TV station, switched to SND for the aural subcarrier, and with AFC off adjusted final-LO coil L6 for maximum meter reading. This yielded correct keypad tuning for all TV channels. However, the indicated frequency was about 0.2 MHz low when tuning manually. Since I was more interested in tuning FM signals than accessing TV channels with the keypad, I reset the LO for accurate manual tuning.

My SAM II had some overtuning. On the low end the frequency went through zero and came back up. On the high end the knob continued to turn after the frequency topped out. I mounted small 10kΩ trimpots to the end lugs of the tuning potentiometer. With these I was able to set the band edges at DC and 300 MHz. This brought the tuning rate down from 40 to 30 MHz per turn in the
FM band.

The SAM II had a number of strong digital spurs below 2 MHz. But the only in-band spur I found was a −5 dBmV signal near 35 MHz from the final LO, plus a little leakage from the calibrator at 150 MHz.

Other SAMs

The SAM Jr is a stripped-down version of the SAM I. It measures signal amplitude only and has no spectrum analyzer or calibrator. An IF bandwidth of 600 kHz makes it unsuitable for use on a crowded FM band. Similar units from other manufacturers use a single-conversion IF near 45 MHz. If the SAM Jr operates this way it would not be easy to modify for a narrower passband.

The SAM III is similar to the SAM II, but the synthesizer can tune any frequency, not just TV channels. The SAM IIID has an RS-232 interface.

The SAM IV is an AC-only SAM IIID with built-in CRT.

The digitally synthesized SAM 1000 and SAM 2000 have 125-kHz frequency steps. This resolution is not suitable for tuning the FM broadcast band with a narrow
IF filter.